Wireless Networking & Mobile Computing CS 752/852 - Spring 2012

Wireless Networking & Mobile Computing

CS 752/852 - Spring 2012

Tamer NadeemDept. of Computer Science

Lec #7: MAC Multichannel

Page 2 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

Multi-Channel MAC for Ad Hoc Networks: Handling Multi-Channel Hidden Terminals Using A Single

Transceiver * (Jungmin So and Nitin Vaidya)

* Slides adapted from J. So


• Multiple Channels available in IEEE 802.11• 3 channels in 802.11b• 12 channels in 802.11a

• Utilizing multiple channels can improve throughput• Allow simultaneous transmissions

Motivation

1

defer

1

2

Single channel Multiple Channels


Problem Statement

• Using k channels does not translate into throughput improvement by a factor of k

• Nodes listening on different channels cannot talk to each other

• Constraint: Each node has only a single transceiver• Capable of listening to one channel at a time

• Goal: Design a MAC protocol that utilizes multiple channels to improve overall performance

• Modify 802.11 DCF to work in multi-channel environment

1 2


802.11 Power Saving Mechanism

• Time is divided into beacon intervals

• All nodes wake up at the beginning of a beacon interval for a fixed duration of time (ATIM window)

• Exchange ATIM (Ad-hoc Traffic Indication Message) during ATIM window

• Nodes that receive ATIM message stay up during for the whole beacon interval

• Nodes that do not receive ATIM message may go into doze mode after ATIM window

Basics


Multi-Channel Hidden Terminals



A

B

C

Time

Beacon

ATIM Window

Beacon Interval



A

B

C

Time

Beacon

ATIM

ATIM Window

Beacon Interval



A

B

C

Time

Beacon

ATIM

ATIM-ACK

ATIM Window

Beacon Interval



A

B

C

Time

Beacon

ATIM

ATIM-ACK

DATA

Doze Mode

ATIM Window

Beacon Interval



A

B

C

Time

Beacon

ATIM

ATIM-ACK

DATA

ACK

Doze Mode

ATIM Window

Beacon Interval



• Consider the following naïve protocol

• Static channel assignment (based on node ID)

• Communication takes place on receiver’s channel• Sender switches its channel to receiver’s channel before transmitting



A B CRTS

A sends RTS

Channel 1

Channel 2



A B CCTS

B sends CTS

Channel 1

Channel 2

C does not hear CTS because C is listening on channel 2



A B CDATA

C switches to channel 1 and transmits RTS

Channel 1

Channel 2

Collision occurs at B

RTS

Related Work

Previous work on multi-channel MAC


Nasipuri’s Protocol

• Assumes N transceivers per host• Capable of listening to all channels simultaneously

• Sender searches for an idle channel and transmits on the channel [Nasipuri99WCNC]

• Extensions: channel selection based on channel condition on the receiver side [Nasipuri00VTC]

• Disadvantage: High hardware cost


Wu’s Protocol [Wu00ISPAN]

• Assumes 2 transceivers per host• One transceiver always listens on control channel

• Negotiate channels using RTS/CTS/RES• RTS/CTS/RES packets sent on control channel• Sender includes preferred channels in RTS • Receiver decides a channel and includes in CTS• Sender transmits RES (Reservation)

• Sender sends DATA on the selected data channel


Wu’s Protocol (cont.)

• Advantage• No synchronization required

• Disadvantage• Each host must have 2 transceivers• Per-packet channel switching can be expensive• Control channel bandwidth is an issue

• Too small: control channel becomes a bottleneck• Too large: waste of bandwidth• Optimal control channel bandwidth depends on traffic load, but difficult

to dynamically adapt

Protocol Description

Multi-Channel MAC (MMAC) Protocol


Proposed Protocol (MMAC)

• Assumptions

• Each node is equipped with a single transceiver

• The transceiver is capable of switching channels

• Channel switching delay is approximately 250us• Per-packet switching not recommended• Occasional channel switching not to expensive

• Multi-hop synchronization is achieved by other means


MMAC

• Idea similar to IEEE 802.11 PSM

• Divide time into beacon intervals

• At the beginning of each beacon interval, all nodes must listen to a predefined common channel for a fixed duration of time (ATIM window)

• Nodes negotiate channels using ATIM messages

• Nodes switch to selected channels after ATIM window for the rest of the beacon interval


Preferred Channel List (PCL)• Each node maintains PCL

• Records usage of channels inside the transmission range

• High preference (HIGH)• Already selected for the current beacon interval

• Medium preference (MID)• No other vicinity node has selected this channel

• Low preference (LOW)• This channel has been chosen by vicinity nodes• Count number of nodes that selected this channel to break ties


Channel Negotiation

• In ATIM window, sender transmits ATIM to the receiver• Sender includes its PCL in the ATIM packet• Receiver selects a channel based on sender’s PCL and

its own PCL• Order of preference: HIGH > MID > LOW• Tie breaker: Receiver’s PCL has higher priority• For “LOW” channels: channels with smaller count have higher

priority

• Receiver sends ATIM-ACK to sender including the selected channel

• Sender sends ATIM-RES to notify its neighbors of the selected channel


Channel Negotiation

A

B

C

DTime

ATIM Window

Beacon Interval

Common Channel Selected Channel

Beacon


Channel Negotiation

A

B

C

D

ATIM

ATIM-ACK(1)

ATIM-RES(1)

Time

ATIM Window

Beacon Interval


Beacon


Channel Negotiation

A

B

C

D

ATIM

ATIM-ACK(1)

ATIM-RES(1)

ATIM-ACK(2)

ATIM ATIM-RES(2)

Time

ATIM Window

Beacon Interval


Beacon


Channel Negotiation

A

B

C

D

ATIM

ATIM-ACK(1)

ATIM-RES(1)

ATIM-ACK(2)

ATIM ATIM-RES(2)

Time

ATIM Window

Beacon Interval


Beacon

RTS

CTS

RTS

CTS

DATA

ACK

ACK

DATA

Channel 1

Channel 1

Channel 2

Channel 2

Performance Evaluation

Simulation ModelSimulation Results


Simulation Model• ns-2 simulator• Transmission rate: 2Mbps• Transmission range: 250m• Traffic type: Constant Bit Rate (CBR)• Beacon interval: 100ms• Packet size: 512 bytes• ATIM window size: 20ms• Default number of channels: 3 channels• Compared protocols

• 802.11: IEEE 802.11 single channel protocol• DCA: Wu’s protocol• MMAC: Proposed protocol


Wireless LAN - Throughput

30 nodes 64 nodes

MMAC

DCA

802.11

MMAC shows higher throughput than DCA and 802.11

802.11

DCA

MMAC

Packet arrival rate per flow (packets/sec) Packet arrival rate per flow (packets/sec)1 10 100 1000 1 10 100 1000

2500

2000

1500

1000

500

Agg

rega

te T

hrou

ghpu

t (K

bps)

2500

2000

1500

1000

500


Multi-hop Network – Throughput

3 channels 4 channels

MMAC

DCA

802.11802.11

DCA

MMAC

Packet arrival rate per flow (packets/sec)1 10 100 1000

Packet arrival rate per flow (packets/sec)1 10 100 1000

Agg

rega

te T

hrou

ghpu

t (K

bps)

1500

1000

500

0

2000

1500

1000

500

0


Throughput of DCA and MMAC(Wireless LAN)

DCA MMAC

3 channels

802.11

MMAC shows higher throughput compared to DCA

6 channels

802.11

3 channels

6 channels

Agg

rega

te T

hrou

ghpu

t (K

bps) 4000

3000

2000

1000

0

4000

3000

2000

1000

0

Packet arrival rate per flow (packets/sec) Packet arrival rate per flow (packets/sec)


Analysis of Results

• DCA• Bandwidth of control channel significantly affects performance• Narrow control channel: High collision and congestion of control

packets• Wide control channel: Waste of bandwidth• It is difficult to adapt control channel bandwidth dynamically

• MMAC• ATIM window size significantly affects performance• ATIM/ATIM-ACK/ATIM-RES exchanged once per flow per beacon

interval – reduced overhead• Compared to packet-by-packet control packet exchange in DCA

• ATIM window size can be adapted to traffic load


Partially Overlapped Channels Not Considered Harmful * (Arunesh Mishra, Vivek Shrivastava, Suman Banerjee, William Arbaugh)

* Slides adapted from Ashwin Wagadarikar, Duke


Spectral Bands and Channels

• Wireless communication uses electromagnetic signals over a range of frequencies

• FCC has split the spectrum into spectral bands• Each spectral band is split into channels

Example of a channel


Typical usage of spectral band

• Transmitter-receiver pairs use independent channels that don’t overlap to avoid interference.

Fixed Block of Radio Frequency Spectrum

Channel A Channel B Channel C Channel D


Ideal usuage of channel bandwidth

• Should use entire range of freqs spanning a channel• Usage drops down to 0 just outside channel boundary

Channel A Channel B

Frequency

Pow

er

Channel C Channel D


Realistic usage of channel bandwidth

• Realistically, transmitter power output is NOT uniform at all frequencies of the channel.

• PROBLEM:• Transmitted power of some freqs. < max. permissible limit• Results in lower channel capacity and inefficient usage of the spectrum

Real Usage

Channel A Channel B

Pow

er

Channel C Channel D

Wastage of spectrum


Consideration of the 802.11b standard

• Splits 2.4 GHz band into 11 channels of 22 MHz each• Channels 1, 6 and 11 don’t overlap

• Can have 2 types of channel interferences:• Co-channel interference

• Address by RTS/CTS handshakes etc.

• Adjacent channel interference over partially overlapping channels• Cannot be handled by contention resolution techniques

Wireless networks in the past have used only non-overlapping channels


Focus of paper

• Paper examines approaches to use partially overlapped channels efficiently to improve spectral utilization

Channel A Channel B

Channel A’


Empirical proof of benefits of partial overlap

• Can we use channels 1, 3 and 6 without interference ?

Ch 1 Ch 6Ch 3

Amount of Interference

Link A Ch 1

Link C Ch 6

Link B Ch 3



• Typically partially overlapped channels are avoided• With sufficient spatial separation, they can be used

Link A Ch 1

Link C Ch 6

Link B Ch 3

Ch 1 Ch 6Ch 3

Virtually non-overlapping



Link A Ch 1

Link B Ch X

Channel Separation

5210

Non-overlapping channels, A = 1, B = 6Partially Overlapped Channels, A = 1, B = 3Partially Overlapped Channels, A = 1, B = 2

Same channel, A = 1, B = 1

LEGEND

3

4

5

6

0 10 20 30 40 50 60Distance between the 2 links (meters)

UD

P Th

roug

hput

(Mbp

s)

• Partially overlapped channels can provide much greater spatial re-use if used carefully!


Interference factor• To model effects of partial overlap, define:

• Interference Factor or “I-factor”

• Transmitter is on channel j• Pj denotes power received on channel j

• Pi denotes power received on channel i

Pi

Pj

I-factor(i,j) =


FcA-11 Mhz-22 Mhz

-50 dB

-30 dB

FcB

Channel AChannel B

Theoretical Estimate for I-Factor

• Theoretically, I-factor = Area of intersection between two spectrum masks of transmitters on channels A and B


Estimating I-Factor at a receiver on channel 6

0 0.2 0.4 0.6 0.8

1

0 2 4 6 8 10 12

Nor

mal

ized

I-fa

ctor

Receiver Channel

I(theory)

I(measured)


WLAN Case study• WLAN comparison between:

• 3 non-overlapping channels, and• 11 partially overlapping channels• over the same spectral band

• WLAN consists of access points (APs) and clients• AP communicates with clients in its basic service set on a single

channel

• GOAL: allocate channels to AP’s to maximize performance by reducing interference


60 60 60 60 60

Why use partial overlap?Consider a case where you have 300 APs

100 100 100

Worst caseInterference by all 100 APs on same channel

Non-overlap3 channels, 100 APs

each

Partial overlap5 channels, 60 APs

each

Worst caseInterference by all 60 APs on same channel + some interference from POV channels


Channel assignment w/ non-overlap

• Mishra et al. previously proposed “client-driven” approach for channel assignment to APs

• Use Randomized Compaction algorithm• Optimization criterion: minimize the maximum interference

experienced by each client

• 2 distinct advantages over random channel assignment:

• Higher throughput over channels• Load balancing of clients among available APs


Channel assignment w/ non-overlap

• (X,C) = WLAN• X = set of APs and C = set of all clients

• How to assign APs to these 3 channels?• MUST LISTEN TO THE CLIENTS!

• To evaluate a given channel assignment• Compute interference for each client:• Sum taken over APs on same channel since channels are

independent• Create vector of cfc’s (CF) and sort in non-increasing order

• Optimal channel assignment minimizes CF

)1)(( xcfc


• Each client builds I-factor model using scan operation• POV(x,xch,y,ych) = 1 if nodes x and y on their channels

interfere with each other• To evaluate a given channel assignment

• Compute interference for each client:• Sum taken over APs that interfere on own channel + all POV

channels• Create vector of cfc’s (CF) and sort in non-increasing order

• Optimal channel assignment minimizes CF

Channel assignment w/ partial overlap

)1)(( xcfc

= +


Results for high interference topologies

• 28 randomly generated topologies with 200 clients and 50 APs– 14 high interference topologies (average of 8 APs in range for

client)– 14 low interference topologies (average of 4 APs in range for client)


Results for low interference topologies

• Using partially overlapped channels and I-factor, clients can experience less contention at the link level. Higher layers have better throughput


Questions

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Wireless Networking & Mobile Computing CS 752/852 - Spring 2012

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